The present invention relates to apparatus and a method for sensing documents, such as banknotes or other security notes, having a pattern printed over at least a part of their surface, to assess the degree of soiling.
It is known to sense the condition of a document by detecting light reflected from its surface and then comparing the intensity of the reflected light with a threshold to determine whether the document should be accepted or rejected. Variations in printed intensity and of ink colour present problems with such a method. Filters may be used to match the colour of the incident light to the ink colour, to suppress the colour information, except where black ink is present on the document. However, this does not give very satisfactory results, in our experience. We have also attempted to measure the degree of soiling by measuring the contrast between elemental areas on a note, again without very satisfactory results.